These days, economic scale of economic developing countries including the so-called “BRICS countries” is rapidly expanding. With this circumstance, global energy demand is also rapidly increasing. So, price of crude oil, natural gas, and so on is rising. Rise of the price of crude oil, natural gas, and so on promotes development and mining of deep oil/gas well, and resources having inferior quality such as sour crude, which did not make no business sense in the past. Therefore, demand of oil well tubular and line-pipe (hereinafter, referred to as “seamless pipe” as a generic name.), which is applicable for deep oil well and sour crude and which comprises a material having high-strength and chemical stability like corrosion resistance, is increasing.
In order to produce a seamless pipe comprising a material having high-strength and chemical stability such as corrosion resistance, high-alloy steel such as 18Cr-8Ni stainless steel (Japanese Industrial Standards: SUS 304) and 13Cr, further, duplex stainless steel (DP steel), and the like has become used instead of using conventional materials like carbon steel and chromium-molybdenum steel.
In DP steel at room temperature, a ferrite phase and an austenitic phase are mixed. When temperature varies from high temperature to low temperature, volume of austenitic phase portion increases to the volume of ferrite phase portion within this crystal structure. Since austenitic phase hardly fuses alloy component in a state of solid-solution, austenitic phase tends to deposit in the boundary with ferrite phase, i.e. crystal grain boundary. It is well known that many cracks and flaws are generated at a time of blooming-rolling and piercing-rolling from the deposition in the grain boundary as a point of origin. The flaw caused by deterioration of the high-temperature ductility is generated by micro destruction in the grain boundary between austenitic phase and ferrite phase. This is because, high-temperature strength of austenitic phase is different from that of ferrite phase and also compounds like sulfide which deteriorates hot workability deposits in the grain boundary.
As a method to reduce flaws on the inner and outer surface of a duplex stainless steel seamless pipe, Patent documents 1 and 2 disclose a method to at least regulate heating temperature to be within a certain temperature range where ferrite ratio becomes appropriate (at a ratio of 30 to 70% without W; 40 to 80% with W). According to these documents, by hot metal working within the temperature range, appropriate hot workability of the material is secured and generation of flaws on the material surface is inhibited.
Moreover, as a countermeasure of generation of flaws attributed to the micro destruction in the grain boundary, other than optimizing heating temperature range, these documents propose methods for inhibiting deposition in the grain boundary by reducing P and S, controlling sulfide form of Ca, Mg, and REM, and adding B.    Patent Document 1: Japanese Patent Application Examined No. 6-89398    Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 9-271811